This invention relates to the manufacture of stator and rotor laminations for use in dynamoelectric machines such as electric motors and more particularly, to an improvement in a die used in the manufacture of such laminations. While the invention is described in detail with respect to lamination manufacture, those skilled in the art will recognize the wider applicability of the inventive principles described hereinafter.
As is well known in the electrical arts, dynamoelectric machines such as electric motors include stator and rotor assemblies which are constructed from stacks of individual laminations formed from suitable electrical material. Typically, the laminations are formed in a multi-station punch press in to which a strip of metal stock is fed. The strip passes through successive work stations where the the stator and rotor laminations are formed in a variety of steps. In producing both types of laminations, one step performed is to stamp out slots or other configurations in the metal to form, for example, a vent, winding receiving slots, or index openings.
A progressive die is used in these stamping operations. The die has a cavity through which a punch travels to strike the material and punch out the appropriate configuration. Stamping of metal parts in this way is a well recognized manufacturing process. One problem occurring when material is punched from a strip is that some of the perforated or punched out material tends to stick to the striking face of the punch and is brought back through the strip. If the material is still clinging to the face during the next stamping cycle, the punch, in effect, will be trying to punch through a double thickness of material. If the material does not stick to the punch, it often falls on the upper surface of the strip. Again, succeeding operations of the punch press will be trying to punch through a double thickness of material. This problem is known in the art as "slugging". Repetitive operation of the die once "slugging" occurs causes a variety of problems. For example, the next operation of the die can improperly deform the work piece. The die itself may be damaged because the punch and die are not designed to stamp a double thickness of material. In any event, down time of the punch press occurs because, at a minimum, the strips being punched must be removed, the damaged portion eliminated and the strip reinserted in the press before operations are again commenced. Such down time occurrences, of course, decrease production and increase cost.
Various techniques have been tried to prevent or control "slugging". Among these are: grinding of the cutting edges of the punches at an angle to the die plate, notching the punches, and dulling the cutting edges of the punches. None of the previous techniques with which I am familiar have been found to be completely successful.